1. Field of the Invention
The present invention relates to a knock control system or detonation control system for a supercharged spark-ignition engine.
2. Description of the Related Art
In gasoline engines, knocking or detonation occurs because of various factors, such as the fuel octane rating, ignition timing, cylinder pressure, engine temperature, intake air temperature, moisture content of intake air, etc..
In supercharged engines, knocking tends to occur more frequently because of the additional air/fuel mixture forced into the engine cylinders and raising of the intake air temperature by adiabatic compression.
Known in the art is a knock control system for preventing or reducing the magnitude of knocking. In this system, a basic spark advance .theta..sub.B is determined in response to an actual engine speed and engine load. A knock sensor senses the occurrence or absence of knocking and increments or decrements a retard correction value .theta..sub.K in response thereto. Then, the retard correction value .theta..sub.K is subtracted from the basic spark advance .theta..sub.B to determine an executive spark advance; .theta..sub.EX .rarw..theta..sub.B -.theta..sub.K. An electronic ignition system is controlled to generate an igniting spark at the executive spark advance .theta..sub.EX. In this way, the spark advance is retarded in such a manner that the frequency and level of knocking are reduced to an acceptable standard.
It is customary to set the basic spark advance .theta..sub.B so that the ignition timing is advanced just before knocking starts to occur when the engine is operated with leaded gasoline. Therefore, when a low octane-rating regular gasoline is used, the abovementioned retard correction value .theta..sub.K is considerably increased so that the spark advance is excessively retarded. This raises the exhaust gas temperature, which adversely affects the exhaust gas purifier device provided in the exhaust system and increases fuel consumption.
To overcome the foregoing disadvantages, a knock control method has been proposed in which the supercharger boosting pressure is lowered when the retard correction value .theta..sub.K exceeds a predetermined reference value of, for example, 5 degrees crank angle (see Japanese Unexamined Patent Publication No. 58-167881). According to this method, the supercharger is overridden when the retard correction value .theta..sub.K becomes greater than 5.degree. CA. This method enables a supercharged engine to be operated with regular gasoline and in a knock control mode while avoiding an undesirable rise in the exhaust gas temperature, because the retard correction value .theta..sub.K is limited to the reference value of 5.degree. CA. When the engine is run on leaded gasoline, the retard correction value .theta..sub.K normally will not exceed 5.degree.CA so that the supercharger continues to operate, and thus a high engine output is maintained.
In this method, however, the determination of whether to reduce or increase the boosting pressure is made without regard to the actual engine load, i.e., the boost pressure is maintained whenever the retard correction value .theta..sub.K is smaller than the predetermined reference value. This brings about the following disadvantages when the engine is run on regular gasoline. During a light load condition, the engine will run without knocking despite the use of regular gasoline. This is because the cylinder pressure is low enough to avoid knocking since the engine cylinders draw only limited amount of air/fuel mixture. Thus, during the light load condition of the engine, the retard correction value .theta..sub.K will be sufficiently decremented to permit supercharging. But, if the engine load is suddenly increased during a transition condition of the engine, knocking will occur since the engine is running on regular gasoline, and the retard correction value .theta..sub.K is therefore increased. As the .theta..sub.K value becomes greater than 5.degree. CA, the supercharger is overridden to lower the boosting pressure. This causes hunting at a boost pressure control valve, such as a waste gate valve in the case of a turbocharger, and a bypass control valve in the case of an engine-driven supercharger. Another disadvantage is that, during the transition condition, there is a considerable time delay before the boost pressure is adequately reduced, because the retard correction value .theta..sub.K can be incremented only gradually. This results in an excessive rise in the exhaust gas temperature and increases the fuel consumption.